The analysis underlying the Tribune's story on U.S. veterans' disability claims is based on data from the U.S. Department of Veterans Affairs that includes more than 3 million claims and nearly 12 million diagnoses as of Jan. 5, 2010.

Because veterans are compensated based on a score measuring their overall disability, or combined disability, the VA does not track the total costs associated with individual ailments or categories of ailments. The Tribune estimated those costs by calculating the percentage that each ailment contributed to a veteran's overall disability rating and applying those percentages to the amount of compensation the veteran receives. Cost figures were then totaled by category for all veterans.

For example, a veteran whose three diagnoses have disability ratings of 40 percent, 20 percent and 10 percent has a combined disability rating of 60 percent. The VA begins to calculate this rating first by sorting the conditions by severity, so that the most disabling ailments are factored first.

In this example, the 40 percent diagnosis is subtracted from 100, which leaves the veteran 60 percent able. The 20 percent diagnosis is then incorporated by multiplying 60 by 0.2 and subtracting the result from 60, which leaves the veteran 48 percent able (60 × 0.2 = 12; 60 - 12 = 48). The final diagnosis of 10 percent is added in a similar way, resulting in a score of 43.2 (48 × 0.1 = 4.8; 48 - 4.8 = 43.2).

The number 43.2 represents the efficiency or ability of this veteran after the approved diagnoses are taken into account. This number is then subtracted from 100, resulting in a combined disability rating of 56.8. The VA rounds this number to 60 because all disability ratings must be in increments of 10 percentage points, according to VA regulations.

Under current compensation rates, the VA would pay this veteran $974 a month. If the veteran has a spouse, children or other dependents, the amount is increased.

To calculate the costs associated with individual diagnoses, the Tribune took the number of percentage points lost to each diagnosis — in this case 40, 12 and 4.8 — and divided it by the total number of points lost, or 56.8. It then applied those percentages to the amount of money paid. For the veteran in this example, the cost of the first diagnosis is estimated at 70 percent of the total cost, or $681.80; the second at 21 percent or $204.54, and the third at 9 percent or $87.66.

In this way, the Tribune could estimate the total amount paid by the VA to veterans suffering from specific illnesses, such as diabetes, or from categories of illness, such as mental disorders.

One wrinkle in this methodology is that if veterans have multiple diagnoses with identical impact on efficiency, it's impossible to know which ailment was used first to calculate the combined disability rating, because the VA keeps no records of this decision in its computer system.

To account for this, the Tribune ran the analysis separately for each body system, such as the respiratory system or the brain. In all cases where a disability related to that body system was tied with a different type of disability, the Tribune gave priority to the different disability. For that reason, estimates of costs related to any body system are conservative.

After discussing the analysis with VA staff, the newspaper made further adjustments. Veterans may receive additional benefits called Special Monthly Compensation for injuries and illnesses that are extremely severe. The VA doesn't track which diagnoses led to the added compensation, potentially affecting the newspaper's analysis. Veterans who receive these payments are flagged in the data, so their additional compensation was removed from the database by subtracting out costs that rose above what the VA normally pays.

The VA also pays some veterans at the 100 percent rating, or $2,673 a month, even though their combined disability rating is less than 100 percent. These veterans have been able to demonstrate that their injuries or illnesses make them unemployable. The VA says these additional amounts, known as Individual Unemployability, may skew the newspaper's estimates. So the Tribune determined which veterans were being paid at the 100 percent level even though their combined rating was lower and subtracted out the additional costs.

World Focus correspondent Mark Litke and producer Ara Ayer travel to Vietnam to report on the long term impacts of Agent Orange/Dioxin on the Vietnamese land and people. This video report was part of their series on New Vietnam.